The purpose of select is to parallelise a few things including reading from the handles identified by its first argument. As such, select should return whenever sysread would return for any of those handles.

sysread obviously returns when provided a write-only handle.

$ perl -E'
    open(my $fh, ">", "file") or die $!;
    my $rv = sysread($r[0], $buf, 100);
    if (!defined($rv)) {
        say "Error: $!";
    } elsif (!$rv) {
        say "eof";
    } else {
        say "Got $rv bytes";
    }
'
Error: Bad file descriptor
[download]

As such, select should do the same.

$ perl -MIO::Select -E'
    open(my $fh, ">", "file") or die $!;
    my @r = IO::Select->new($fh)->can_read();
    my $rv = sysread($r[0], $buf, 100);
    if (!defined($rv)) {
        say "Error: $!";
    } elsif (!$rv) {
        say "eof";
    } else {
        say "Got $rv bytes";
    }
'
Error: Bad file descriptor
[download]

You're saying select should block forever in the event of an error even though sysread would not. That makes no sense.

Awesome, and very helpful, thanks!